Gas seals are currently used widely to seal process gases in large rotary machines such as turbo compressors and associated turbo machinery. The industry has adopted as its standard, an arrangement of sealing cartridge which is comprised of a stacked assembly that can be slipped on to the end of the rotor.
The most common type of gas seal in current use is a tandem seal, as shown in section in FIG. 1 of the accompanying drawings. A similar seal is shown, for example, in FIG. 3 of U.S. Pat. No. 5,412,977. The cartridge comprises two stages, namely an inboard stage 10 and an outboard stage 12. The inboard stage 10 is used to seal the complete process gas pressure. The outboard stage 12 is also engineered to be capable of sealing the complete process gas pressure. However, during normal running, it is only exposed to around 5–15 bar of gas pressure, its main function being to act as a back up to the inboard stage 10. Should the inboard stage 10 suffer catastrophic failure during operation, the outboard stage 12 takes on the full sealing function and enables the compressor to be run down, stopped and the process gas pressure vented in a controlled manner.
Each stage 10, 12 comprises a primary ring 14 mounted in a retainer and balance diameter assembly 18 in the machine casing. A spring 20 within the assembly 18 acts on the primary ring 14 through a pressure plate 22 that is sealed relative to the stationary assembly housing. The spring 20 urges the primary ring towards a mating ring 16 that rotates with the machine rotor 30.
The primary ring 14 has axial end faces which are exposed to the pressure of the process gas and have different surface areas. The primary ring 14 therefore acts as a differential piston on which the pressure of the process gas exerts a force in the direction to compress the spring 20. In operation, an equilibrium occurs between the gas pressure forces and the spring force when there is a small gap between the primary and the mating rings 14 and 16. This gap prevents the wear to the surfaces of the rings 14 and 16 while limiting escape of the process gas to a small and acceptable leakage.
The manner in which a gas seal operates and the details of its construction, such as the choice of materials to make the rings, are well known in the art and need not therefore be described further in the context of the present invention.
In the known seal cartridges, the two sealing stages are mounted on a common support sleeve 40. The inner surface of the sleeve 40 and the outer surface of the rotor 30 are stepped so as to limit the extent to which the sleeve can move to the left, as viewed in FIG. 1, relative to the rotor 30. A single locknut 42 on the rotor 30 is therefore all that is required to clamp the support sleeve 40 on the rotor 30. This allows both the sealing stages 10 and 12 to be released from the machine by removal of one locknut 42. At one end, the support sleeve 40 has a radial flange 44 which carries the mating ring 16 of the inboard stage 10. The mating ring 16 is held against the flange 44 of the support sleeve 40 by a spacer sleeve 50 which itself has a radial flange 52 which carries the mating ring 16 of the outboard stage. An O-ring seal 54 seals between inner surface of the spacer sleeve 50 and the outer surface of the support sleeve 40. A locking sleeve 60 clamps the mating ring 16 of the outboard stage 12 against the flange 52 of the spacer sleeve 50 and its inner surface is also suitably sealed relative to the outer surface of the spacer sleeve 50.
The above described construction of a tandem seal is convenient in that it allows for easy replacement of both stages of the seal and it enables a compact construction in that the entire cartridge is held in place by only one locknut.
While the tandem seal is useful it has shortcomings and disadvantages. As earlier explained, it is quite common for only one stage to fail but the known construction does not allow the different stages to be worked on independently. Thus, in the embodiment illustrated in FIG. 1, in order to access the inboard stage, it is necessary to dismantle the entire outboard stage, which may not itself require attention. Many other problems can be identified, and are discussed in more detail below, which stem from the fact that the two stages cannot function separately.